"He's ... pressing [Iraqi Prime Minister Nouri al-Maliki] to crack down on, as one official put it, 'the nearly daily Iranian flights over Iraqi territory to Syria.' The U.S. accuses Iran of shipping weapons to Bashar al-Assad's regime and is warning Iraq that these shipments are fueling the conflict and are dangerous to the region."

The Associated Press reports the overflights "have long been a source of contention between the U.S. and Iraq." Iraq promised to inspect the flights last year, the AP says, but an official says only two have been checked since then.

"The air corridor over Iraq has emerged as a main supply route for weapons, including rockets, antitank missiles, rocket-propelled grenades and mortars, as well as Iranian personnel, according to American intelligence officials. There are supply lines on the ground as well.

"Iran has as an enormous stake in Syria, which is its staunchest Arab ally and has provided a channel for Iran's support to the Lebanese Islamist movement Hezbollah."